Your level is stored in a signed integer format, which has a maximum of value of 2,147,483,647. Up to and including level 399 the XP required to get to the next level is 55*CL^2, where CL is your current level. Based on that formula you should be able to get to level 489. But you can't get to level 401 because at exactly level 400 the formula no longer applies, and the required XP for a level up is set to 2,147,483,647. I don't believe this is a bug, but I also don't see why someone would have done it deliberately.